Among the various types of optical data storage media are a group of media which can be both recorded on and erased. The signals which such media can produce vary among three general types: reflective, transmissive and absorptive. These signals may be produced in a variety of ways, including the use of multilayer dye-polymer materials, where the signals appear as pits (or bumps) or holes; phase-change materials where the signal is represented by a localized amorphous region in an otherwise crystalline layer or vice-versa, the transition being induced by heat; and magneto-optical materials, where the signals are localized regions which bear a magnetization in a direction different from that of the surrounding regions.
In each of these materials, formation of the signals (i.e., recordation of the data) includes the localized application of heat, accomplished by the absorption of light from a laser focused on the data track. Erasure is also achieved by the application of heat, either to be absorbed by the same layer which absorbed the heat during recordation, or by another layer in the medium, depending on the technology of the medium. In two-layer dye-polymer media, for example, a retention layer when at ambient temperature rigidly retains the surface protrusions induced by a neighboring expansion layer during recordation, but when selectively heated above its glass transition temperature during the erase process, permits the expansion layer to resume its uniform unexpanded state, thereby removing the data. In magneto-optical media, erasure is effected by heating the data-bearing layer in the presence of a magnetic field which again changes the direction of magnetization in the data regions to return the entire layer to a uniform magnetization direction. Finally, in phase-change media, erasure is effected by heating the data-bearing layer to a temperature just below its melting point, then cooling it to regain its original crystalline state.
Heretofore, such erasure has been achieved with the use of a laser focused on the data track, with the data bits passing over the point of focus in succession. Information is thus erased at the same speed as it is recorded or played back. The process is timeconsuming and tedious, and requires tracking and focusing considerations similar to those encountered during recordation and reading.